This invention relates to analog-to-digital converters, and more particularly, to an optical encoder apparatus for indicating revolutions of a shaft.
Optical encoders are commonly used to convert shaft positional information into digital signals. The information provided by the encoder may represent the angular position of the shaft or the number of revolutions made by the shaft. In the former case, the encoder includes a complex code member which designates or defines a number of discrete shaft positions, commonly referred to as digit positions, and the encoder provides a different multibit binary-coded word for each digit position. The presence of a complex code member permits shaft position encoders to take into account interdigital positions so as to provide an unambiguous coding of shaft position information.
Shaft position encoders, on the other hand, are of simple construction, including a single sensor, a light source and a shutter rotated by the shaft. The shutter is adapted to normally interrupt or block the illumination of the sensor by light from the light source, but to permit illumination of the sensor during a short time for each revolution of the shutter. Upon illumination, the sensor provides a change in its output state. The number of times that the output state changes over a given period is a representation of the number of complete revolutions of the shaft during that period.
Ideally, for single sensor encoders, the output state of the sensor will change only once each revolution, and under such ideal condition, the encoder would provide an unambiguous indication at the completion of each revolution of the shaft. However, various conditions, either mechanical or electrical in nature, may cause multiple false counts in single sensor encoders. For example, in a multi-dial utility meter register which measures usage of a commodity such as gas, water or electricity, the shafts are rotated at a very slow speed through the use of reduction gear train, and the rotational speed of the shafts varies with changes in demand of the commodity. Under such conditions, shutter backlash may cause multiple outputs to be provided during one revolution. Other factors contributing to multiple false counts are ambient light leakage and device tolerances, particularly when semiconductor light sensing devices are used.